1. Field of the Invention
The present invention relates to a polymer electrolyte for a fuel cell, a method for producing them, and a fuel cell.
2. Description of the Related Art
Due to the recent attention to various environmental problems, a new energy technique has thus attracted special interests. It is such a fuel cell technique which is considered a promising new energy technique and it is anticipated that it will become one of the most important technologies in the future. A polymer type fuel cell using a proton-conductive polymer as an electrolyte has attracted such special interest because of features such as good operation property at low temperatures, possibility of miniaturization, creating light, etc.
An example of a polymer electrolyte for a polymer type fuel cell, is Nafion (trademark of Du Pont Co.) which has ultrastrong acid groups containing fluorinated polymer. However, Nafion is very expensive because it is a fluorinated polymer, and control of water must be strictly followed because of its low water retention in the case of using it as a fuel cell. Furthermore, it is necessary when using a fluorine containing compound to take the environment into consideration with respect to synthesis and disposal. Therefore, a polymer electrolyte of a non-fluorinated proton-conductive material is desirable by today's market standard.
Some studies have already been made with respect to the non-fluorinated polymer-based proton-conductive polymer. For example, a fuel cell using a sulfonated aromatic polyether ether ketone as the electrolyte has been researched. A detailed report on the synthesis of the sulfonated aromatic polyether ether ketone and its characteristics are disclosed in Polymer, 1987, Vol. 28, 1009. This report discloses that an aromatic polyether ether ketone, which is insoluble in organic solvent, becomes soluble in an organic solvent by highly sulfonating it, thereby facilitating film formation. The hydrophilic nature of these sulfonated aromatic polyether ketone has increased thus the water-solubility or a reduction in strength on water absorption occurred. When the water-solubility occurs, a fuel cell normally forms water as a by-product as a result of a reaction between fuel and oxygen, the sulfonated aromatic polyether ketone is therefore not suitable to use as an electrolyte for a fuel cell. The Japanese Patent Kokai Publication No. 6-93114, with respect to an electrolyte of a sulfonated aromatic polyether ether ketone, shows that the inventors succeeded in the production of an electrolyte having excellent strength through a process of introducing a crosslinkable functional group at the time of polymer synthesis and crosslinking this functional group after film formation.
Polymeric Material Science and Engineering, 68, 122-123 (1993) and U.S. Pat. No. 5,271,813 disclose that sulfonated compounds of polymers (e.g. sulfonated aromatic polyether ether ketone, sulfonated aromatic polyether sulfone, etc.) can be utilized as an electrolyte of a device for electrolysis of water (UDEL P-1700 used as polyether sulfone is a polymer classified as polysulfone (PSF)). However, there is no description about various physical properties such as primary structure or ion-exchange group equivalent weight of these sulfonated compounds of polymers, and it is difficult for the resulting sulfonated polysulfone not to dissolve in water due to its high water absorption.
These conventional techniques have problems such as an expensive electrolyte, insufficient strength, difficult synthesis of a polymer material, etc.